gSi • Experiments ou the Reflexion of Heat. 



of a grate oppofite which it was placed. At a diftance of 3- inches from the fpeculum, on 

 the refle£ling fide of it, was placed a thermometer, and another clofe byit^out of the 

 courfe of the reflefted rays. The whole was guarded in front againft the influence of the 

 fire by an oaken board, i| inch thick, in which was a circular opening of i^ inch diameter, 

 oppofite the plain mirror, to permit the fire to fliine upon it- There was alfo a wooden 

 partition between the mirror and the thermometers in which was a hole that allowed the 

 rays to fall on the firfl mentioned thermometer, but prevented their accefs to the latter. 

 When this apparatus was expofed to the fire the firft thermometer rofe 7 degrees in five 

 minutes, while the other indicated a change in the temperature of the place, amounting to 

 no more than half a degree. 



Experiment 6. ReJleBlon of fire-heat by a prifm. When the lafl: defcribed experiment was 

 repeated by fubfl;ituting inftead of the mirror a prifm whofe angles were 90° ; 45°; and 45°, 

 fo placed that the radiation from the fire fhould pafs through one fide, and make its 

 incidence on the hypothenufe within the prifm, the rife was 45 degrees in 1 1 minutes} but 

 the temperature of the place, as (hewn by the other thermometer, having been raifed- 1~ de- 

 grees in the fame time, the efFeft of the refleftion was only 2^ degrees. 



Experiment 7. RefieStion of invifible filar heat. On a board of about 4 feet, 6 inches 

 long, a fmall plain mirror was fixed at one end, and at the other, two thermometers, as 

 {hewn in fig. 2. PI. XVI. where for the convenience of infertion the board is reprefented 

 as broken. The diftance of one thermometer from the face of the mirror was 3 feet cl- 

 inches ; and the other thermometer was put at the fide of it facing the fame way, but out 

 of the courfe of the rays, that were to be reflected from the mirror. The colours of the 

 ' prifm were thrown on a flieet of paper, having parallel lines drawn upon it at half an inch 

 from each other. The mirror was fl;ationed upon the paper; and was adjufted in fuch a 

 manner as to prefent its polifhed furface in angle of 45°, to the incident coloured rays, by 

 which means they would be refle£ted towards the ball of the firft mentioned thermometer. 

 In. this arrangement tlie whole apparatus might be withdrawn from the colours to any re- 

 quired diftance, by attending to the laft vifible red colour, as it fliewed itfelf on the lines of 

 the paper. When the thermometers were properly fettled to the temperature of their fituation, 

 during which time the mirror had been covered, the apparatus was drawn gently away from 

 the colours fo far as to caufe the mirror, which was now open, to receive only the invifible 

 rays of heat which lie beyond the confines of the red. The refult was, that in ten minutes 

 the firft thermometer received four degrees of heat refledled to it in the ftridleft optical 

 manner, and the great regularity with which thefe invifible rays obeyed the law of reflexion 

 was fuch that the very fenfible thermometer, which was chofen as a ftandard, and was 

 within an inch of the other, remained all the time without the leaft indication of any change. 

 The mirror was then taken away, and in ten minutes the meafuring thermometer again loft 

 the heat it had required.. This therefore, as the Dodlor obferves, is a mojl decifive experiment ' 

 in proof of the exifience of itvvifibU ra-js^ of their being fubje51 to the laws of refiellion^ani of their 

 power ofoccaftoning heat. 



Experiment 



